There are two parts to diagnosing a personality disorder - first, a person has to have all the aspects of a PD. From the DSM V:

The essential features of a personality disorder are impairments in
personality (self and interpersonal) functioning and the presence of
pathological personality traits. To diagnose a personality disorder,
the following criteria must be met:
A. Significant impairments in self (identity or self-direction) and
interpersonal (empathy or intimacy) functioning.
B. One or more pathological personality trait domains or trait facets.
C. The impairments in personality functioning and the individual‟s
personality trait expression are relatively stable across time and
consistent across situations.
D. The impairments in personality functioning and the individual‟s
personality trait expression are not better understood as
normative for the individual‟s developmental stage or sociocultural
environment.
E. The impairments in personality functioning and the individual‟s
personality trait expression are not solely due to the direct
physiological effects of a substance (e.g., a drug of abuse,
medication) or a general medical condition (e.g., severe head
trauma).

If you have all that stuff (as determined by a doctor - again, people are never a good judge of themselves because they overestimate somethings and overlook others, especially the requirement that there are no other attributable causes) then you have a personality disorder.

Then to find out which of the PD's you have, you need to have all (not some) of the aspects of the type, and it must be acute and consistent. If you don't have all the requirements, then you don't have that PD even if you have most of the traits.

For example, these are the requirements for AvPD per the DSM V:

The essential features of a personality disorder are impairments in
personality (self and interpersonal) functioning and the presence of
pathological personality traits. To diagnose avoidant personality
disorder, the following criteria must be met:
A. Significant impairments in personality functioning manifest by:
1. Impairments in self functioning (a or b):
a. Identity: Low self-esteem associated with self-appraisal
socially inept, personally unappealing, or inferior;
excessive feelings of shame or inadequacy.
b. Self-direction: Unrealistic standards for behavior
associated with reluctance to pursue goals, take personal
risks, or engage in new activities involving interpersonal
contact.
AND
2. Impairments in interpersonal functioning (a or b):
a. Empathy: Preoccupation with, and sensitivity to, criticism
or rejection, associated with distorted inference of others‟
perspectives as negative.
b. Intimacy: Reluctance to get involved with people unless
being certain of being liked; diminished mutuality within
intimate relationships because of fear of being shamed or
ridiculed.
B. Pathological personality traits in the following domains:
1. Detachment, characterized by:
a. Withdrawal: Reticence in social situations; avoidance of
social contacts and activity; lack of initiation of social
contact.
b. Intimacy avoidance: Avoidance of close or romantic
relationships, interpersonal attachments, and intimate
sexual relationships.
c. Anhedonia: Lack of enjoyment from, engagement in, or
energy for life‟s experiences; deficits in the capacity to feel
pleasure or take interest in things.
2. Negative Affectivity, characterized by:
a. Anxiousness: Intense feelings of nervousness, tenseness,
or panic, often in reaction to social situations; worry about
the negative effects of past unpleasant experiences and
future negative possibilities; feeling fearful, apprehensive,
or threatened by uncertainty; fears of embarrassment.
C. The impairments in personality functioning and the individual‟s
personality trait expression are relatively stable across time and
consistent across situations.
D. The impairments in personality functioning and the individual‟s
personality trait expression are not better understood as normative
for the individual‟s developmental stage or socio-cultural
environment.
E. The impairments in personality functioning and the individual‟s
personality trait expression are not solely due to the direct
physiological effects of a substance (e.g., a drug of abuse,
medication) or a general medical condition (e.g., severe head
trauma).